Speak Up! Costa Rican Bats Use Leaves as Hearing Aids

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Bats in Costa Rica have evolved a neat trick to help them hear
their roost-mates flying above: They use leaves to funnel sound
in a natural version of an old-timey ear horn.

The Spix's disc-winged bat
(Thyroptera tricolor), named for suction-cuplike discs
on its wings and feet, is found in South America. Unlike other
cave-dwelling bat species, disc-winged bats roost each day in the
unfurling leaves of plants outside of caves. These leaves form a
tube shape as they go from folded-up to flat, meaning the bats
can roost only for a day before having to find another leaf in
the proper shape.

Spix's disc-wing bats are also cliquish. They form groups of five
or six and stay together, despite their nightly evictions.

"They can stay together for many, many years, which is quite rare
in bats," said Gloriana Chaverri, a postdoctoral researcher at
Boston University and the director of research and conservation
at the nonprofit Costa Rican ProSur Foundation. "These bats are
fascinating." [ See
Images of the Leaf-Living Bats ]

Chatting bats

A previous study of the bat's chattering calls revealed that
despite the need to recognize roost-mates,
roosting bats weren't great at discerning whether they were
talking to a close buddy or a stranger. The finding got Chaverri
and her colleague Erin Gillam of North Dakota State University
wondering whether the roosting leaves were changing the sound
of the calls. Because the leaves are shaped like megaphones, the
researchers also wondered if they might amplify the roosting
bats' cries.

To find out, the researchers recorded calls from bats, including
79 "inquiry" calls, simple one-note cries used by flying bats to
try to locate roosting pals. The other 65 recorded calls were
complex "response" calls, which can consist of as many as 20 to
25 sounds made in response to an inquiry call, Chaverri said.

The researchers then played the response and inquiry calls from
either inside the leaf or outside the leaf, posting a microphone
at the opposite end to catch the sound.

Can you hear me now?

They discovered the leaves don't make very good megaphones: The
calls of the roosting bats were only amplified by a decibel or
two, an inaudible difference for human ears and unlikely to make
a big difference in how far a bat can be heard.

But the funneling effect of the leaf inward made a big difference
for how well roosting bats could hear their flying friends.
Compared with a
bat call heard without the leaf, a call heard from within the
leaf was up to 10 decibels louder.

Decibels are measured on a logarithmic scale, such that 60
decibels is half as loud as 70 decibels, so as few as 10 decibels
can make a big difference. In the case of the bats, the acoustic
effect could increase the distance at which a flying bat is heard
by 65 feet to 98 feet (20 meters to 30 meters).

However, the boosted cries were distorted, Chaverri said, because
not all frequencies of sound amplify equally. The finding
explains why roosting bats can hear their friends, but not
necessarily recognize them. As a result, bats in the roost cry
out in response to any inquiry they hear. It's the job of the
flying
bat to recognize the complex response call as familiar and
join the roost. The findings are surprising, Chaverri said,
because while animals are known to use structures such as logs to
boost their calls, there are fewer examples of animals detecting
calls with the aid of an object.

The researchers report their findings today (Oct. 15) in the
journal Proceedings of the Royal Society B.